1. Field of the Invention
This invention relates to a process for the manufacture of pyrazine by catalytic dehydrocyclization of diamines. This process has experienced a problem of low selectivity which has remained unresolved in many cases.
2. Description of the Background
Copper-chromite catalysts have been used for this reaction. Additionally, other methods for the preparation of pyrazine are also known.
Korean Pat. No. 82-719 describes that 3.2 grams of alkylpyrazine was obtained after 1.5 hours of reaction time from 5 grams of N-(2-alkylhydroxy)ethylenediamine by the direct dropping of the reactant which was vaporized in to top of reactor, followed by extraction with ether, drying with anhydrous magnesium sulfate, and evaporating the ethylether. The catalyst was activated at 350.degree. C. with 4 liters of air for 2 hours followed by 1 liter of hydrogen for 1 hour. The process has the disadvantage of low production capacity, because of 0.17 hour.sup.-1 of weight hourly space velocity, which is the feed rate of reactant in weight per hour to the unit weight of catalyst. Also, a low yield of 80% was obtained. As disclosed in Japan Pat. (kokai) No. 55-122,769, the catalyst was activated by treating with a hydrogen stream diluted with nitrogen for 18 hours at a relatively low temperature of 170.degree.-200.degree. C. The catalyst was pelletized CuO--Cr.sub.2 O.sub.3 of 1 to 1.25 weight ratio (CuO to Cr.sub.2 O.sub.3) with 5% of pulverized carbon and water. It has the advantages of a lower treating temperature for the catalyst and a reaction temperature of 265.degree.-300.degree. C., but the disadvantages of a long treatment time with hydrogen, a low conversion of 82%, and a low yield of 78%.
Conventional methods for the preparation of pyrazines from different reactants are disclosed in the following patents: German Pat. No. 2,722,307; Japanese Pat. No. 49-25,947; Japan Pat. (kokai) No. 49-30,383; Japan Pat. (kokai) No. 54-27,577; and Appl. Catal., 29.161 (1987). The conventional methods of preparing pyrazines described hereinabove have disadvantages of obtaining low yield. This implies that piperazines and multi-alkylated (more than 2) pyrazines may be formed together, and also pose difficulties in separation because of difficult control of the distillation process due to a high portion of by-products.